This invention relates to devices for giving displays by varying the luminescence efficiency of fluoroescent materials utilizing the electric field effect of changing the molecular orientation of liquid crystals and provides novel and useful compositions relating to liquid crystal materials therefor. Generally known as electro-optical effects of liquid crystals are dynamic scattering effect, twisted nematic electric field effect, guest-host effect, cholesteric-nematic phase transition effect, etc. These electro-optical effects provide displays by the scattering or absorption of ambient light, as distinct from the effects of luminescent materials which per se product luminescent displays such as lamp displays, light emitting diode displays, electroluminescence displays, plasma displays or the like. The displays resorting to the electro-optical effects are advantageous over those of the latter type in that they involve reduced energy consumption but have the drawback of lacking brilliance.
The present invention has overcome the above problem and provides novel useful display devices and compositions therefor which have the low power consumption characteristics of so-called passive displays not luminescent in themselves and which incorporate a self-luminescent component for giving a brilliant active display.
R. D. Larrabee has already proposed to add a fluorescent material to a liquid crystal material and vary the fluorescent intensity of the material by electric field (RCA Review, Vol. 34, p 329, 1973). However, his paper states that he failed to find liquid crystal materials which do not absorb ultraviolet light at room temperature. This appears attributable to the fact that when causing a fluorescent material in a liquid crystal material to absorb a varying amount of light in accordance with the orientation of the liquid crystal to vary the fluorescence intensity with the light absorption, the exciting light is absorbed by the liquid crystal layer without effectively exciting the fluorescent material. U.S. Pat. No. 3,960,753 further discloses fluorescent liquid crystal compositions comprising 4'-alkyloxy- (or -acyloxy-)benzylidene-4-cyanoaniline as a liquid crystal material, but the operating temperature of the liquid crystal material is considerably higher than room temperature. U.S. Pat. No. 3,844,637 similarly discloses fluorescent liquid crystal compositions comprising 4'-methoxy- (or -ethoxy-)benzylidene-4-n-butylaniline as a liquid crystal material. However, the compositions per se absorb violet light or near ultraviolet light without permitting effective excitation of the fluorescent material.
This invention provides novel luminescent display devices of the low power consumption type with use of liquid crystal materials which form a nematic, smectic or cholesteric phase at room temperature and which are usable in display devices for electronic apparatus or the like and entirely different from those of the foregoing references, the liquid crystal materials permitting effective excitation of fluorescent materials.